1. Field of the Invention
The present invention relates to an electric power steering apparatus of the ball screw type, and more particularly to an electric power steering apparatus of the ball screw type which prevents the load from being concentrated on the ball located at the outermost of the ball nut within the inner peripheral screw groove with the ball nut bended, thereby preventing the durability of the ball nut and the ball from being deteriorated.
2. Description of the Prior Art
A general electric power steering apparatus includes a steering system extending from a steering wheel to two wheels, and an auxiliary power mechanism for supplying auxiliary steering power to the steering system.
The auxiliary power mechanism includes a torque sensor for detecting a steering torque applied to a steering wheel by the driver and outputting an electric signal proportional to the detected steering torque, an electronic control unit (ECU) for generating a control signal based on the electric signal transferred from the torque sensor, a motor for generating auxiliary steering power based on the control signal transferred from the electronic control unit, and a belt type power transmission unit for transmitting auxiliary power generated by the motor to a rack bar via a belt.
An electric power steering apparatus of a ball screw type as the auxiliary power mechanism may include a return tube type, a deflector type, an end cap type and so on.
FIG. 1 is a sectional view schematically illustrating the electric power steering apparatus of the ball screw type according to the related art, and FIG. 2 is a sectional view schematically illustrating a portion of the electric power steering apparatus of the ball screw type according to the related art.
As illustrated in the drawings, the rack-driven auxiliary power steering apparatus according to the related art includes a driving unit 120 for generating a power to be controlled by an electronic control unit (ECU) which determines the degree of the steering of a steering wheel 113 by a torque sensor 111 mounted to a top end of a steering column 110, a rack bar 130 for making a tie rod 133 connected to a wheel moved according to the degree of the steering of the steering wheel 113, and a driven unit 140 for transforming the receiving rotating force of the driving unit 120 to an axially moving force and then transmitting it to the rack bar 130.
Here, the driving unit 120 includes an electric motor 121 controlled by an electronic control unit (ECU), a driving pulley 123 coupled to a shaft of the electric motor 121, and a drive belt 125 wound on a driving pulley 123.
The driven unit 140 also includes the ball nut 141 supporting the rack bar 130 within a rack housing surrounding the rack bar 130, and the driven pulley 143 coupled to an outer peripheral surface of the ball nut 141.
The driving pulley 123 connected to the shaft of the electric motor 121 and the driven pulley 143 connected to the ball nut 141 are disposed parallel to each other, the driving belt 125 is mounted on the driving pulley 123 and the driven pulley 143 to transmit a rotating force of the electric motor 121 to the rack bar 130, and the rack bar 130 is moved leftward and rightward by an operation of the ball nut 141 to generate an auxiliary steering force.
An inner peripheral screw groove 201 having a gothic arc-shaped cross-section is spirally formed on an inner peripheral surface of the ball nut 141, an outer peripheral screw groove 203 having a gothic arc-shaped cross-section is spirally formed on an outer peripheral surface of the rack bar 130, a ball is inserted between the inner peripheral screw groove 201 and the outer peripheral screw groove 203. While contacting the inner peripheral screw groove 201 and the outer peripheral screw groove 203, the ball 210 moves along the inner peripheral screw groove 201 and the outer peripheral screw groove 203, rolls from one side to the opposite side, and circulates through the ball circulation passage 205 formed between the end cap 220 and the ball nut 141 coupled to two sides of the inner peripheral surface of the ball nut 141 respectively.
In the rack-driven auxiliary power steering apparatus according to the related art, as there is generally equally formed a center radius of the ball (R, i.e., distance from the axis of the ball nut to a center of the ball) moving along the inner peripheral screw groove 201, the load is concentrated on the ball located at the outermost of the ball nut with the longitudinal direction within the inner peripheral screw groove with the ball nut bended, thereby causing the durability of the ball nut and the ball to be deteriorated.
Further, there is a step between the inner peripheral screw groove and an inlet of the end cap (i.e., part where the ball escapes the inner peripheral screw groove and enters the end cap) at a location where they meet with each other, thereby causing noise and vibrations to be generated as soon as the ball moving along the inner peripheral screw groove from one side to the opposite side enters the inlet of the end cap.